1. Field of the Invention
The present invention relates to an illumination device configured to combine lights from multiple light sources using rod integrators, and in particular to an illumination device preferably used in a high-intensity projection-type display apparatus.
2. Description of Related Art
Projection-type display apparatuses can be sorted roughly into portable-type small-sized devices, medium-sized devices for small and medium-sized conference rooms, and high-intensity large-sized devices for use in large conference rooms, movie theaters, and the like. Among these, in particular, the large-sized devices cannot be replaced with flat displays.
In the configuration of such large-sized devices, a high pressure mercury lamp is used as a light source with large output and a small light-emitting unit. A xenon light source is used in a large-sized device whose intensity is particularly high. However, the latter light source has a large bulb, while the light-emitting part is small, and the luminous efficiency is low, so that a large power source is necessary. Accordingly, the entire device is very large, and thus its usage is limited. For such a reason, commercial high-intensity apparatuses achieve high intensity by using a plurality of high pressure mercury lamps that can be comparatively smaller in size.
For the high-intensity devices thus provided with a plurality of light sources, one of the measures to achieve still higher intensity is to increase the power of the light sources. However, adopting of such a measure is not easy since light source development, cooling means development, and the like are necessary. Another measure to achieve high intensity is to improve combining efficiency when combining lights from multiple light sources. This measure is important as a measure to be selected since high intensity can be achieved with the same power by effectively using light that is wasted due to conversion into heat.
Various inventions for improving the combining efficiency of combining lights from multiple light sources have been made. The following are main examples of improvements.
JP 2006-308778A discloses a configuration in which a deflecting member is provided on the entrance side of a rod integrator that combines light from a plurality of light sources, and makes the illumination distribution uniform. The deflecting member transmits and deflects light in such a manner that system optical axes from the light sources just before entering the rod integrator are perpendicular to the entrance surface. A refractive-type prism is used as the deflecting member.
Further, U.S. Pat. No. 7,033,056 discloses a configuration in which a combination of a sub-rod integrator and a rectangular prism is provided on the entrance side of a main rod integrator that combines light from a plurality of light sources, and makes the illumination distribution uniform. This rectangular prism reflects light in such a manner that system optical axes from the light sources just before entering the rod integrator are perpendicular to the entrance surface.
However, these conventional examples have problems such as the following from the viewpoint of practical use.
In the case of the example disclosed in JP 2006-308778A, the refraction prism exit faces are disposed in an inclined manner relative to the entrance surface of the rod integrator. Accordingly, some of the light that is condensed in the vicinity of the center of the entrance surface of the rod integrator, with which a wide gap is formed, will enter through a refraction surface for the light from a different light source. As a result, such light does not effectively reach an illumination region, and becomes loss light.
Further, although the rectangular prism in the example disclosed in the U.S. Pat. No. 7,033,056 has a size of several millimeters in fact, the entrance and exit surfaces thereof need reflection prevention multi-coating, and the reflection surface thereof needs heat-resistant multilayer film mirror coating. With this size, generally a process is performed in a manner that a prism is created in a large size, and grinding is performed after coating, because if coating is performed after grinding, a shadow of the coating jig remains. That is, when coating is performed as the latter step, there is no waste portion after coating, and thus grinding cannot be performed. Accordingly, a shadow of the coating jig will remain somewhere. Also, there is a problem that the produced prisms are expensive.